The present invention relates to flash evaporation, and more particularly, to an improved vertical tube flash evaporator apparatus and method of flash evaporation utilizing same.
Flash evaporators produce steam by dropping the pressure on water at the saturation temperature. The excess heat flashes part of the water into steam. Flash evaporators of various types have been utilized extensively for many years to vaporize liquids. The conventional uses of flash evaporation are usually directed to vaporizing part or all of the solvent from a solution. A major use of the process is to purify or gather a desired material, either the solvent or the solute. Very little effort has been directed to utilizing the vapor pressure from flash evaporation for production of power.
Since the primary use of flash evaporation has been material separation or distillation, the evaporation effectiveness in relation to the power input required has not been a significant concern. In recent years, however, renewed interest has been directed to utilization of the heat stored in relatively low temperature (less than boiling point in normal atmospheric conditions) water bodies to generate power. Heat is usable by its transfer to a colder body. For example, as early as 1900, D'Arsonval proposed that power could be generated by utilizing the warm surface water of the ocean and the cold water from the deep. However, when the differential between the respective temperatures of the hot and cold body is small, it is more difficult and less efficient to produce power.
Georges Claude and Paul Boucherot made the most significant pioneer efforts in producing power from seawater approximately 50 years ago as described in their U.S. Pat. No. 2,006,985, issued July 2, 1935. They describe a process and apparatus that depends on flash evaporating a small amount of the warm water from the surface of the ocean to obtain a low pressure vapor or steam. This low pressure steam was used to turn a turbine in the process of cooling, and it was then condensed on tubes through which cold water from the deep was passed. Most of the current work and proposals for generating power from the heat stored in seawater are variations of Claude's work, known as ocean thermal energy conversion (OTEC). In practical applications, huge quantities of water must be flowed through the power generating system to obtain sufficient energy for power production because of the small temperature difference (approximately 15.degree.-20.degree. C.) between the warm surface layer of the ocean water and the cold deep water. Therefore, because of many energy requirements in related machinery, and because of many losses, the practical efficiency probably obtainable in such OTEC systems is in the 2-3% range. Further, the amount and cost of equipment required always increases greatly with a decrease of the temperature difference in the water utilized. Consequently, the heat available in this relatively low temperature seawater can be converted to power only with a large, costly plant, at a very low efficiency, and by handling extremely large amounts of cold seawater to absorb the heat. Therefore, even seemingly small increases in efficiency of the component parts and processes of the energy conversion system can pay huge dividends in significantly reducing the size and cost of such systems.
The current projected designs for a power production plant for an OTEC system will require a large, low pressure vapor turbine, which is as yet unbuilt and untested in the United States. It is anticipated, however, that when sufficient design efficiencies of the remaining components of this system have been developed, the large, low pressure vapor turbine will be economically feasible.
One of the components of the system in which increased efficiencies pay significant dividends is in the effective flash evaporation of the warm sea water. Claude and Boucherot used a vertical tube bubbler in their apparatus, and they also disclosed a falling jet and screen arrangement in their U.S. Pat. No. 2,006,985. Subsequent developments, usually in the material purification or distillation industry, focused on increasing surface area of the vaporizing liquid, such as by flowing films of the liquid over the surfaces of a plurality of parallel, spaced-apart vertical plates. The height of development of flash evaporators utilizing this technique for OTEC systems is illustrated by the controlled flash evaporation process and apparatus devised by Ralph C. Roe and Donald F. Othermer as described in their article, "Controlled Flash Evaporation," 93 Mechanical Engineering Journal 27-31 (1971). They described the inefficiencies of conventional multiple state flash evaporation processes in which bubbles of vapor almost explode from the turbulent surface. The controlled flash evaporation system they described as being much more efficient than conventional systems utilized films of liquid descending in a rolling flow on the inside surface of a vertical tube to vaporize quietly, practically in equilibrium, and at nearly the same temperature as the vapor formed. Roe and Othermer claimed this system eliminates losses due to turbulence.
In contravention of the trend toward laminar film flow over surfaces practically in equilibrium to quietly vaporize liquids, the inventor of this present invention has recognized that maximizing vaporization depends not only on maximizing liquid surface area, but also on constantly renewing the surface properties of the liquid. For example, evaporation on the liquid surface quickly cools the surface to the point where further vaporization in actually inhibited. Maintenance of this cool surface alone, such as in the quiet, laminar flow systems, cannot maximize vaporization. On the contrary, it is necessary to renew the vaporization surface properties by circulating warmer liquid from the inside to the outside layers or by stripping away the cooled external surface liquid to expose warmer liquid to the surface.